The subject matter of the invention relates to railway trucks, especially a type of truck sometimes referred to as a bogie, i.e., an assembly comprising a basic three-piece frame structure including a bolster and a pair of side frames, with two wheelsets mounted by means of bearings to the side frames.
The invention is concerned with dramatically changing the inter-axle parameters of such railway trucks.
In each wheelset, the wheels are fixed on the axles. The truck frame and thus the truck as a whole is mounted for pivotal or swiveling motion with respect to the frame of the car with which the truck is associated, for which purpose, the bolster of the truck is connected with the body of the car by means of a central upright pivot.
In the past, railway cars have been provided with a wide variety of trucks. For many years, the wheelsets were mounted in the truck side frames by means of journal ("plain") bearings. However, in recent years, the journal bearings have been replaced by roller bearings, and the present day conventional truck is known as the AAR three-piece roller bearing freight car truck.
Prior to the introduction of roller bearings, it was common to incorporate spring planks, i.e., a generally long flat metal or wood plank extending transversely between the two side frame members and positioned so that the ends of the plank extend between the side frame members and the springs. Their position was usually maintained primarily by the weight of the car. Sometimes, interfitting means on the plank and the side frames were provided in order to keep the plank from working out of place in service.
In plain bearing trucks, such spring planks were found to have some utility in limiting parallelogramming of the side frames in curves and to position the side frames in the roll direction. However, such spring planks are not effective in preventing the small angle parallelogramming motions involved in truck hunting at high speeds on tangent track, and this has been found to be a serious disadvantage.
Spring planks have also been employed in certain passenger and transit car trucks in combination with a "swing hanger" support from a rigid truck frame to supply a "soft" lateral suspension. In this case, it is the rigid truck frame and the axle bearing/truck frame interface rather than the spring plank which is involved in determining whether the truck has a hunting problem.
The use of spring planks in freight car trucks was generally discontinued prior to the introduction of roller bearings for several reasons. For example, it was observed that the friction between the spring plank and the side frames contributed to the axle misalignment problem on tangent track by capturing the parallelogrammed position occurring in curves. This causes the wheels to run with an angle to the track centerline for long distances following curves. Moreover, because of the lack of isolation of the spring plank from wheel/rail impacts, breakage of the spring plank was common.
In plain, i.e., journal bearing trucks, the elimination of the spring plank did not result in a significant amount of truck hunting, although wheelset hunting was rather common. This usually occurred within the lateral clearance of the journal and did not result in any severe periodic lateral motion of the truck frame or of the car body. However, with the introduction of the roller bearing, truck hunting became a major problem. It was also observed that wheel flange wear was more severe with roller bearing trucks. At the time of adoption of the roller bearings, it was not fully appreciated that the plain or journal bearing had permitted a small amount of inter-axle steering.
There have been many unsuccessful attempts to solve the truck hunting problem which accompanied the introduction of roller bearings. One of these was to introduce resilient pads to simulate the lateral clearance previously present with plain bearings. Experiments were also made with spring planks alone. These were more promising, but they were not significantly effective, and they provided no relief for the wheel/rail wear problem because of the complete lack of any axle steering capability.
Through the use of analytical tools and techniques developed in recent years, we now understand the reasons why resilience was not a substitute for the bearing clearance and tended to make matters worse. We also now understand why the use of spring planks alone tended to introduce as many problems as it solved. Various of these reasons are brought out herebelow.
It is first noted that the AAR roller bearing truck currently in general use has several serious shortcomings which are as follows.
Firstly, the truck framing is subject to a parallelogramming motion in which there is yaw motion between the side frames and the bolster, and the side frames can move relatively to each other in the fore-and-aft direction. Secondly, the wheelsets are prevented from engaging in significant steering motions with respect to each other because of the large friction forces which are present between the roller bearing adapters and the side frames. Thirdly, because of the crude or inaccurate assembly tolerances commonly present in conventional trucks, and because of the action of the brake shoe forces, there is likely to be a small axle-to-axle angle present at all times.
Because of the parallelogramming motion, truck hunting is widespread, especially with lightly loaded cars. Truck hunting causes excessive wear of many truck and car body components, and it will cause significant damage to certain types of lading. Furthermore, these trucks tend to retain the parallelogrammed position after leaving a curve, and both axles tend to run with a substantial wheel/rail angle of attack for long distances on tangent track, even if the assembly tolerances are good.
The prevention of the steering of one axle relative to another within the truck makes it impossible for the leading axle to attain a radial position in curves, and this is a serious disadvantage. It also prevents the axles from steering away from assembly errors to a parallel position on tangent track.
All of the above shortcomings of the three-piece roller bearing trucks are effectively remedied by the constructions described in my prior patents No. 4,131,069 and No. 4,455,946, above identified. The effectiveness of the arrangements of those prior patents has been established by extensive field testing showing that the flange wear rate is characteristically reduced to one-third of the wear rate in conventional trucks. Moreover, reductions in the average rolling resistance in travel of the vehicle have also been found to be as large as 30 percent. The basic theoretical approach to choosing the amount of parallelogramming restraint and steering flexibility applied to the inter-axle motions of trucks embodying the inventions of the two patents just identified is summarized in a technical paper by Marcotte, Caldwell and the present applicant, which was presented to the Winter Annual Meeting of the American Society of Mechanical Engineers in 1978. This paper defines two key parameters, the "Stiffness Ratio, R" and the "Normalized Yaw Stiffness", and describes how these parameters affect truck stability and curving. A value for "R", the ratio of the inter-axle steering (Normalized Yaw) stiffness to the parallelogramming stiffness, between 0.5 and 1.0 has been found to give the best combination of stability and curving. A low value for the Normalized Yaw stiffness is needed for good curving. With the steering arm constructions of the '069 and '946 patents, it is relatively easy to provide the desirable low value for both of these parameters. Without steering arms, only less desirable values for "R" greater than 1.0 are available.
The type of analysis outlined by the paper above referred to also sheds considerable light on the shortcomings of the conventional AAR truck and the lack of success of the earlier efforts to correct those shortcomings. For example, the conventional AAR truck has an "R" value of about 10, which is very poor. Adding resilient pads alone to conventional roller bearing trucks increases the value for "R", making truck hunting worse. Adding a spring plank alone will reduce "R" and reduce truck hunting, and it will prevent parallelogramming of the truck frame and limit the wheel/rail angle of attack on tangent track, but without resilient pads, the lack of axle steering makes curving very poor and makes the tangent track performance less desirable than it could be.